First you need to understand that prices for printing will be different in different areas.
With that said, my printer charges a setup fee of $15 per color, per screen. So you've got 3 different locations, with 2 colors on 2 of them and 1 color on the other. That would be $75 in setup fees.
Then they would charge the general public $2.25 for 1 color and $2.50 for 2 colors in a run of 20 or 50 shirts. With the 3 locations, printing 20 shirts would be $145 plus a $75 setup fee=$220.
For the now 50 shirts they are "thinking" about it would be $362 plus a $75 setup fee=$437.
Also, it's important to note that price can be a factor for customers, but it's not always the only factor. When I first began looking for a contract printer, I got contract pricelists and quotes from several different printers. Some of them had great communication, nice looking shops and what I considered reasonable prices.
But one of them had horrible communication, no experience working with brands, extremely unflexible, and their prices were way out of line from the other printers I had contacted...until they found out that I was moving my office across the street from their shop. Then they wanted to be VERY accomodating and have someone with a brand and continual work so close. Too late!
They turned me off by wanting to charge me almost twice what everyone else was charging. Most of the prices were within a couple of dollars of each other. But this one was about $5 higher per shirt. That's a lot of money.
Not everybody's trying to lowball you. Sometimes if you're not in line with what they really can get some other places, why would they pick you?